hi steve,
i wouldn't hesitate to call your gull a good caspian. bill and scaps are absolutely perfect for a 3cy cach, it just has a rel. strongly marked tail, but that's well inside variation for casp.
edit: now, after having a second look, there might be some reason for argentatus-genes in it: 1. all coverts are brownish immature, not even median coverts are grey (mc are usually the firsts to get adult grey), 2. the very well marked uppertail- and undertail coverts. now there could be some discussion whether this is within caspian variation or not - actually with so many hybrids around it is hard to prove so, but from my experience variation is huge in caspian (as in other LWHG) especially when it comes to 2-4cy birds. it might be a retarded and well marked pure female caspian (structure, p10-mirror, tertials, bill coloration - everything fits) or it may be a hybrid or backcross, hard to tell.
p.s. a great and well ordered gullery you have on your website, did i mention that yet?
all the best,